Surface-modified single-walled carbon nanotubes and methods of detecting a chemical compound using same

ABSTRACT

A method for surface modification of single walled carbon nanotubes. In one embodiment, the method includes the steps of providing a detergent solution, adding a plurality of single walled carbon nanotubes into the detergent solution, performing a first sonication to disperse the single walled carbon nanotubes in the detergent solution, and performing a second sonication after the first sonication to make detergent encased single walled carbon nanotubes. At least one of the plurality of single walled carbon nanotubes is at least partially wrapped by one or more detergent molecules to make it a detergent encased single walled carbon nanotube. In one embodiment, the detergent comprises SDS, PSS or a combination of them.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit, pursuant to 35 U.S.C. §119(e), ofprovisional U.S. patent application Ser. No. 60/603,181, filed Aug. 20,2004, entitled “Surface-Modified Single-walled Carbon Nanotube OpticalBiosensors and Methods of Making and/or Using Same,” by Wei Zhao andChulho Song, which is incorporated herein by reference in its entirety.

Some references, which may include patents, patent applications andvarious publications, are cited in a reference list and discussed in thedescription of this invention. The citation and/or discussion of suchreferences is provided merely to clarify the description of the presentinvention and is not an admission that any such reference is “prior art”to the invention described herein. All references cited and discussed inthis specification are incorporated herein by reference in theirentireties and to the same extent as if each reference was individuallyincorporated by reference. In terms of notation, hereinafter, “[n]”represents the nth reference cited in the reference list. For example,[14] represents the 14th reference cited in the reference list, namely,Zhao, W.; Song, C.; Pehrsson, P. J. Am. Chem. Soc. 2002, 124,12418-12419.

This invention was made with certain Government support, and theGovernment has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to surface modified single walled carbonnanotubes and a method of detecting a chemical compound using same.

BACKGROUND OF THE INVENTION

There is great interest in using single-walled carbon nanotubes (SWNTs)as nanoscale probes and sensors in biological electronics and opticaldevices because the electronic and optical properties of SWNTs areextremely sensitive to the surrounding environmental changes[1-5,14-18,21-25,30,31,35,42-45,50,51]. To date, most research on SWNTshas focused on electronic devices, with relatively little work onoptical biosensors. In order to use SWNTs as optic biosensors, someimmediate questions needs to be solved such as how the sensors respondto chemical variables like pH [5c] and concentration of glucose,ethanol, various ions, or proteins.

SWNTs are a collection of semiconducting, metallic nanotubes and amixture of them in different diameters that can be probed by variousspectroscopic methods including Raman spectroscopy and UV/vis/NIRabsorption spectroscopy. Raman spectroscopy can be used to determinemany aspects of an SWNTs sample, including size distribution, disorderfrom defects or functionalization, and general electronic behaviors.

SWNTs possess unique optical properties as a result of theirone-dimensional nature. Sharp peaks in the density of states, called vanHove singularities (VHS), arise from a quantization of the electronicwave vector in the 1-D system [26]. As a result of these singularities,SWNTs possess peaks in their optical spectra that correspond tointerband transitions from the valence band to the conduction band. Inaddition, the transitions are found to be grouped in spectral spaceaccording to nanotube type (metallic vs. semiconducting) and band index,which are responsible for the observed sharp and pronounced opticalabsorption peaks in individual HiPco SWNTs [21, 23].

The side view of an SWNT 100 is illustrated in FIG. 1 a. The SWNT has afirst end 110, an opposite, second end 120 and a body portion definedtherebetween the first end 110 and the second end 120. The body portioncontains a carbon “wall” that is formed by a plurality of carbon atomsin certain arrangements as known to people skilled in the art. Asillustrated in FIG. 1 b, the SWNT 100 can be considered to have anexterior surface 130, an interior surface 140, and a cavity 150,respectively.

Because current techniques produce SWNTs in a mixture form with aboutone third of metallic nanotubes and two thirds of semiconductingnanotubes [46], separations of semiconducting SWNTs from metallic SWNTsare required for practical applications [6,47]. The study of SWNTseparations is a subject of intense exploration [18-20]. The discoveryof surfactant-assisted dissolution of SWNTs in aqueous sodium dodecylsulfate (SDS) solution [23] has greatly stimulated the progress in thisexciting area [18-20].

Water-soluble SWNTs (ws-SWNTs) with undisrupted characteristic opticalabsorption features have been obtained by surface modifications such asfunctionalization with carboxylate groups [14] and surface coatings withsurfactants [21, 23] or single stranded DNA [18, 19]. FIG. 1 cillustrates an SWNT encased in polymeric material 170. It has beenobserved that the optical characteristics of surface modified SWNTs arepH sensitive [14, 18, 23-25], which suggests new opportunities for SWNTsbased optical biosensor applications yet to be explored. Nanotubes mayeven be combined with recently developed nanolasers [33], nanowaveguides [53] and nano optical fibers [34], to make opticalnanosensors in the near future.

Therefore, a heretofore unaddressed need exists in the art to addressthe aforementioned deficiencies and inadequacies.

SUMMARY OF THE INVENTION

The present invention, in one aspect, relates to a method for surfacemodification of single walled carbon nanotubes. The method includes thesteps of providing a detergent solution, adding a plurality of singlewalled carbon nanotubes into the detergent solution, performing a firstsonication to disperse the single walled carbon nanotubes in thedetergent solution, and performing a second sonication after the firstsonication to make detergent encased single walled carbon nanotubes. Atleast one of the plurality of single walled carbon nanotubes is at leastpartially wrapped by one or more detergent molecules to make it adetergent encased single walled carbon nanotube. In one embodiment, thedetergent comprises SDS, PSS or a combination of them. The firstsonication process is performed at a frequency in the range of from 0 to20 kHz for a time period of from 0 to 5 minutes. The second sonicationprocess is performed at a frequency in the range of from 20 to 200 kHzfor a time period of from 0 to 15 minutes. Each of the first and secondsonication processes is performed at a frequency for a time period suchthat no significant amount of defects that may affect the opticalproperties of the single walled carbon nanotubes is introduced.

In one embodiment, at least one optical property of the detergentencased single walled carbon nanotubes responds to a chemical propertychange in the solution of the detergent encased single walled carbonnanotubes. The single walled carbon nanotubes comprise semiconductingnanotubes, metallic nanotubes or a combination of them. The response ofthe at least one optical property of the detergent encased single walledcarbon nanotubes to the chemical property change of the solution of thedetergent encased single walled carbon nanotubes is more sensitivelyrelated to the semiconducting nanotubes than the metallic nanotubes inthe solution of the detergent encased single walled carbon nanotubes.The response of the at least one optical property of the detergentencased single walled carbon nanotubes to the chemical property changeof the solution of the detergent encased single walled carbon nanotubesis reversible.

In another aspect, the present invention relates to a biosensorresponsive to a chemical property in an environment. The biosensor has aplurality of single walled carbon nanotubes forming an array and showinga dependence of the chemical property. The biosensor also has aprocessor coupled to the array of the plurality of single walled carbonnanotubes for processing the response of the plurality of single walledcarbon nanotubes to the chemical property. At least one of the pluralityof single walled carbon nanotubes is at least partially wrapped by oneor more detergent molecules to make it a detergent encased single walledcarbon nanotube. In one embodiment, the detergent comprises SDS, PSS ora combination of them.

In one embodiment, the chemical property is a hydrogen peroxideconcentration in an environment, and the detergent encased single walledcarbon nanotube is optically responsive to the hydrogen peroxideconcentration in the environment.

In another embodiment, the at least one detergent encased single walledcarbon nanotubes is further wrapped by one or more enzyme molecules toform a solution of detergent encased single walled carbon nanotubes withthe enzyme. The hydrogen peroxide may be produced by an enzyme as one ofthe turnover products from a corresponding substrate.

In yet another embodiment, the chemical property is glucoseconcentration in an environment, the detergent encased single walledcarbon nanotube is further wrapped by one or more glucose oxidase thatmay covert the glucose to hydrogen peroxide and gluconic acid, and theat least one detergent encased single walled carbon nanotube withglucose oxidase is optically responsive to hydrogen peroxide that isproduced from the glucose by glucose oxidase in the environment.

In yet another aspect, the present invention relates to a surfacemodified single walled carbon nanotube that has a layer of carbon atomsforming a wall defining a cavity therein. The wall as formed has anouter surface and an inner surface, and a first end and an opposite,second end and at least one molecule non-covalently attached at least toone of the inner surface and the outer surface of the single walledcarbon nanotube. The single walled carbon nanotube is at least partiallysurface modified with the at least one molecule to show an opticaldependence of a chemical property of an environment. The at least onemolecule comprises one of SDS, glucose oxidase, single stranded DNA,double-stranded DNA and PSS. The chemical property is one of pH value,hydrogen peroxide concentration, glucose concentration and ethanolconcentration of the environment.

In a further aspect, the present invention relates to a method ofdetecting a chemical compound. The method includes the steps ofproviding a solution of surface modified single walled carbon nanotubes,associating the solution of surface modified single walled carbonnanotubes with the chemical compound, and detecting optically a chemicalproperty change of the solution of surface modified single walled carbonnanotubes corresponding to the chemical compound so as to detect thechemical compound. In one embodiment, the detergent comprises SDS, PSSor a combination of them. The associating step comprises a step offorming a solution of the surface modified single walled carbonnanotubes and the chemical compound.

In one embodiment, the chemical compound comprises at least one of abase and acid, and the corresponding chemical property is pH of thesolution of the surface modified single walled carbon nanotubes.

In another embodiment, the chemical compound is hydrogen peroxide, andthe corresponding chemical property is hydrogen peroxide concentrationin the solution of the surface modified single walled carbon nanotubes.

In yet another embodiment, the method further comprises the step ofadding an amount of glucose oxidase to the solution of the surfacemodified single walled carbon nanotubes before the associating step sothat at least one of the plurality of surface modified single walledcarbon nanotubes is further wrapped by one or more glucose oxidasemolecules. The chemical compound is glucose, and the correspondingchemical property is glucose concentration in the solution of thesurface modified single walled carbon nanotubes with glucose oxidase.The glucose oxidase may convert glucose to hydrogen peroxide andgluconic acid, and the optically detecting step comprises a step ofmeasuring the optical properties of the solution of the surface modifiedsingle walled carbon nanotubes with glucose oxidase responsive to theconcentration of the hydrogen peroxide that is produced from glucose byglucose oxidase in the solution of the surface modified single walledcarbon nanotubes with glucose oxidase.

In one embodiment, the method further comprises the step of adding anamount of enzyme to the solution of the surface modified single walledcarbon nanotubes before the associating step so that at least one of theplurality of surface modified single walled carbon nanotubes is furtherwrapped by one or more of the enzyme molecules. The chemical compound isa substrate of the enzyme that is convertable to hydrogen peroxide asone of its turnover products by the enzyme, and the correspondingchemical property is the substrate concentration in the solution of thesurface modified single walled carbon nanotubes with the enzyme.

In another embodiment, the chemical compound is iodine, and thecorresponding chemical property is the iodine concentration in thesolution of the surface modified single walled carbon nanotubes. In yetanother embodiment, the chemical compound is oxidant, and thecorresponding chemical property is the oxidant concentration in thesolution of the surface modified single walled carbon nanotubes. In oneembodiment, before the associating step, the method further comprisesthe steps of adding an amount of glucose oxidase to the solution of thesurface modified single walled carbon nanotubes so that at least one ofthe surface modified single walled carbon nanotubes is further wrappedby one or more glucose oxidase molecules and adding an amount of iodideto the solution of the surface modified single walled carbon nanotubeswith glucose oxidase. In one embodiment, the chemical compound is iodinethat is produced in situ from the reaction of iodide with hydrogenperoxide, which is produced from glucose by the glucose oxidase, and thechemical property is the iodine concentration in the solution of thesurface modified single walled carbon nanotubes.

In one aspect, the present invention relates to a method of opticallydetecting a chemical property change in a solution of surface modifiedsingle walled carbon nanotubes induced by sonication. The methodincludes the steps of providing a solution of surface modified singlewalled carbon nanotubes, performing a sonication on the solution ofsurface modified single walled carbon nanotubes, and detecting opticallythe response of the solution of surface modified single walled carbonnanotubes to a chemical property change of the solution of the solutionof surface modified single walled carbon nanotubes induced by thesonication. In one embodiment, the detergent comprises SDS, PSS or acombination of them. The sonication is performed at a frequency in therange of from 20 to 200 kHz for a time period of from 0 to 200 minutesat a temperature in the range of from 0 to 100° C. The chemical propertyis pH in the solution of surface modified single walled carbonnanotubes. The chemical property change is corresponding to nitrous acidand nitric acid concentrations induced by sonication in the solution ofsurface modified single walled carbon nanotubes.

These and other aspects of the present invention will become apparentfrom the following description of the preferred embodiment taken inconjunction with the following drawings, although variations andmodifications therein may be affected without departing from the spiritand scope of the novel concepts of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. shows (a) a side view of a single walled carbon nanotube; (b) aperspective view of a single walled carbon nanotube; (c) SWNT encased inpolymeric material; and (d) SWNT encased in SDS.

FIG. 2. illustrates schematically a process to synthesize SDS encasedSWNTs.

FIG. 3. shows the pH dependence of SDS encased SWNTs.

FIG. 4. shows the pH dependence of pristine SWNTs with coating of TritonX-100 and PVP.

FIG. 5. shows the pH dependence of pristine SWNTs with coating ofpoly(sodium 4-styrenesulfonate (PSS).

FIG. 6. illustrates the chemical structure of the polymers used asmodels for pH sensing according to one embodiment of the presentinvention.

FIG. 7. shows a possible mechanism of the formation of nitrous acid andnitric acid in aerated water under sonication.

FIG. 8. is a flowchart for a process to measure the optical response ofSDS-SWNTs to pH change induced by sonication according to one embodimentof the present invention.

FIG. 9. shows the absorption spectra of (a) a 5 mL HiPco SWNT solution;and (b) a 0.5 mL HiPco SWNT solution after different sonication timesaccording to one embodiment of the present invention.

FIG. 10. shows the changes in pH of an SDS solution as a function ofsonication time according to one embodiment of the present invention.

FIG. 11. shows (a) an enlarged absorption spectra showing the details inthe S₁₁ region with corresponding pH changes induced by sonication ofFIG. 9 a and (b) absorbance dependence of two S₁₁ peaks of the HiPcoSWNT solution on different pHs induced by sonication.

FIG. 12. shows the absorption spectra of a 0.5 mL HiPco SWNT solutionsonicated for 0.5 minutes, 65.5 minutes, and the recovery of the S₁₁peaks after the pH is adjusted to 10.

FIG. 13. shows the absorption spectra of a 0.5 mL SDS-encased HiPco SWNTsolution in a pH 6.0 phosphate buffer (0.05M) before and after anextensive sonication.

FIG. 14. is a flowchart for a process to measure the optical response ofSDS-SWNTs to H₂O₂ at different concentrations according to oneembodiment of the present invention.

FIG. 15. is a flowchart for a process to measure the optical response ofSDS-SWNTs to H₂O₂ at different time points.

FIG. 16. shows time-dependent vis/NIR absorption spectra of SDS-SWNTs inpH 6.0 buffer solution after addition of 30 ppm H₂O₂.

FIG. 17. shows (a) concentration-dependent vis/NIR absorption spectra ofSDS-SWNTs in pH 6.0 buffer solutions under different H₂O₂ concentrationsfrom 1 ppm to 200 ppm; (b) the absorbance of the S₁₁ band of a SDS-SWNTssample at peaks 1245 nm and 1322 nm normalized by the absorbance of theS₂₂ band of the same sample at peak 659 nm, A_(S) ₂₂ /A_(S) ₁₁ ,responding to H₂O₂ concentration changes exponentially.

FIG. 18. shows the reversible optical response of a H₂O₂ interactedSDS-SWNTs sample after removal of H₂O₂ by MnO₂ catalyzed decomposition.

FIG. 19. shows the spectral changes of an SDS-encased HiPco SWNTsolution at different catalase concentrations.

FIG. 20. shows the recoverable optical absorption of an H₂O₂-interactedSDS-HiPco SWNT solution in pH 6.0 buffer after decomposing the H₂O₂ intoH₂O and O₂ with catalase (140 units/ml).

FIG. 21. shows (a) absorption spectra of an SDS-encased HiPco SWNTsolution containing 100 ppm H₂O₂ at various pHs; (b) the ratio As₁₁/As₂₂of the absorbances of the S₁₁ (1322 nm) and the S₂₂ (659 nm) peaksreversibly responds to the pH changes.

FIG. 22. is a flowchart for a process to measure the optical response ofSDS-SWNTs to pH in the presence of a fixed amount of H₂O₂.

FIG. 23. shows the absorption spectra of a SDS-encased HiPco SWNTsolution in a pH 6.0 buffer in response to I₂. The suppressed spectralfeatures are restored by titrating the solution to pH 10.6.

FIG. 24. is a flowchart for a process to measure the optical response ofGOx-SDS-SWNTs to glucose at different time points.

FIG. 25. is a flowchart for a process to measure the optical response ofGOx-SDS-SWNTs to glucose at different glucose concentrations.

FIG. 26. is a flowchart for a process to measure the optical response ofGOx-SDS-SWNTs with a fixed amount of glucose to pH changes.

FIG. 27. shows (a) absorption spectra of SDS-encased HiPco nanotubes ina pH 6.0 buffer solution before and after addition of 40 units/mLGO_(x); (b) absorption spectra of GO_(x)-SDS-SWNTs (HiPco) in a pH 6.0buffer solution change with time after addition of 0.5 mM glucose.

FIG. 28. shows the recovery of the optical absorption ofglucose-interacted GO_(x)-SDS-SWNTs (HiPco) samples in pH 6.0 bufferswith 40 units/mL GO_(x) by the addition of (a) catalase; (b) adjustingthe pH to 10.0.

FIG. 29. shows (a) changes in the absorption spectra of GO_(x)-SDS-SWNTs(HiPco) suspended in 1 wt % SDS solution versus glucose concentration;(b) glucose concentration-dependent absorbances; (c) mechanisticillustration of the process of detecting glucose with GOx-SDS-SWNTs.

FIG. 30. shows (a) changes in the absorption spectra of GO_(x)-SDS-SWNTsin a 1 wt % SDS solution as a function of time after addition of 1.0 mMglucose; (b) as a function of time at various glucose concentrations.

FIG. 31. shows (a) the aqueous solutions of DNA and DNA-SWNTs; (b) Ramanspectra of a pristine HiPco mat sample and a DNA-SWNTs sample drop-driedon a Si substrate.

FIG. 32. shows (a) UV/vis/NIR absorption spectra of a DNA-SWNTs solutiontitrated from pH 7.0 to 10.0; (b) the pH dependence of the absorbance ofthe (8,7) nanotube S₁₁ band at 1280 nm of a DNA-SWNTs sample normalizedby the absorbance of the S₂₂ band at 730 nm of the same sample, A_(S) ₁₁(1280)/A_(S) ₂₂ (730).

FIG. 33. shows UV/vis/NIR absorption spectra of the DNA-SWNTs solutionshown in FIG. 32 titrated (a) from pH 10.0 down to 5.5; (b) from pH 6.0up to 9.0.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is more particularly described in the followingexamples that are intended as illustrative only since numerousmodifications and variations therein will be apparent to those skilledin the art. Various embodiments of the invention are now described indetail. Referring to the drawings, like numbers indicate like partsthroughout the views. As used in the description herein and throughoutthe claims that follow, the meaning of “a,” “an,” and “the” includesplural reference unless the context clearly dictates otherwise. Also, asused in the description herein and throughout the claims that follow,the meaning of “in” includes “in” and “on” unless the context clearlydictates otherwise.

DEFINITIONS

The terms used in this specification generally have their ordinarymeanings in the art, within the context of the invention, and in thespecific context where each term is used. Certain terms that are used todescribe the invention are discussed below, or elsewhere in thespecification, to provide additional guidance to the practitionerregarding the description of the invention. For convenience, certainterms may be highlighted, for example using italics and/or quotationmarks. The use of highlighting has no influence on the scope and meaningof a term; the scope and meaning of a term is the same, in the samecontext, whether or not it is highlighted. It will be appreciated thatthe same thing can be said in more than one way. Consequently,alternative language and synonyms may be used for any one or more of theterms discussed herein, nor is any special significance to be placedupon whether or not a term is elaborated or discussed herein. Synonymsfor certain terms are provided. A recital of one or more synonyms doesnot exclude the use of other synonyms. The use of examples anywhere inthis specification, including examples of any terms discussed herein, isillustrative only, and in no way limits the scope and meaning of theinvention or of any exemplified term. Likewise, the invention is notlimited to various embodiments given in this specification.

As used herein, “around”, “about” or “approximately” shall generallymean within 20 percent, preferably within 10 percent, and morepreferably within 5 percent of a given value or range. Numericalquantities given herein are approximate, meaning that the term “around”,“about” or “approximately” can be inferred if not expressly stated.

As used herein, the term “SDS” refers to sodium dodecyl sulfate.

As used herein, the term “PSS” refers to poly(sodium4-styrenesulfonate).

As used herein, the term “PVP” refers to poly(vinylpyrrolidone).

As used herein, the term ”GOx” refers to glucose oxidase.

As used herein, the term “SWNTs” refers to single walled carbonnanotubes.

As used herein, the term “ws-SWNTs” refers to water soluble singlewalled carbon nanotubes.

As used herein, the term “SDS-SWNTs” refers to SDS encased single walledcarbon nanotubes.

As used herein, the term “GOx-SDS-SWNTs” refers to SDS encased singlewalled carbon nanotubes with glucose oxidase.

As used herein, the term “GOx-SDS-HiPco” refers to SDS encased HiPcosingle walled carbon nanotubes with glucose oxidase.

As used herein, the term “DNA-SWNT” refers to double stranded DNAencased single walled carbon nanotubes.

As used herein, the term “UV/vis/NIR” refers to ultraviolate-visible-near infra red.

OVERVIEW OF THE INVENTION

Among other things, applicants have invented a method of detectinghydrogen peroxide with SDS encased single walled carbon nanotubes andcorresponding biosensor(s). In one aspect, the present invention relatesto near IR optical absorption (or reflection) methods for any forms ofSWNTs, which can be isolated nanotubes, aggregated nanotubes or bundles.The SWNTs can be any SWNTs made by various techniques including HiPcoand SWNTs made by a laser oven technique and by an arc dischargetechnique. Charge groups of modification species on the sidewall ofSWNTs are required for pH sensing in aqueous solution. Without thecharge groups' presence, SWNTs will not work for sensing pH changes.

In one embodiment, the present invention utilizes near IR opticalabsorption (or reflection) methods that use the intensity ratio S₁₁/S₂₂of the first and second optical interband transitions of semiconductingSWNTs for sensing [14]. The S₂₂ band is less sensitive to environmentchanges so it can be used to serve as an internal reference. Inapplications, two wavelengths near the peak absorption of S₁₁ and S₂₂coming from two near IR laser beams or from filtered light generatedfrom a white light source can be adopted. No visible light is requestedfor excitation, which could be harmful for biological applications. Fromthis discovery, a wide range of sensors can be designed by using a widerange of materials with charge groups such as amines, proteins and DNA.The negatively charged sulfonate groups of PSS and phosphate groups ofDNA serve as sensing groups for pH. A device can be built by combiningindividual or bundled SWNTs with nanolasers [32, 33] or nano opticalfibers [34] so the whole device can be a nanodevice.

METHODS AND IMPLEMENTATIONS

Without intent to limit the scope of the invention, additional exemplarymethods and their related results according to the embodiments of thepresent invention are given below. Note that titles or subtitles may beused in the examples for convenience of a reader, which in no way shouldlimit the scope of the invention. Moreover, certain theories maybeproposed and disclosed herein; however, in no way they, whether they areright or wrong, should limit the scope of the invention so long as thepresent invention is practiced without regard for any particular theoryor scheme of action.

EXAMPLES Example 1 SDS Encased SWNTs and Their Properties Preparation ofSDS Encased SWNTs

In one embodiment, about 2.4 mg pristine SWNTs (such as HiPco orTube@Rice) were weighed on a TGA microgram balance and placed in a 10 mLtest tube with 5 mL 1 wt % SDS aqueous solution. In an ultrasonic bath(Branson Model 1510, 42 kHz), a mild (first) sonication was applied for1-3 minutes to disperse HiPco nanotubes and then the mixture wasvigorously sonicated (a second sonication) for about 1 minute. Shortsonication time was applied because the optical properties of HiPcoSWNTs are very sensitive to sonication. The resulting mixture wascentrifuged (Sargent-Welch Scientific Co.) for about 1 hour. 0.8 mL ofthe top portion of the centrifuged sample was decanted and diluted withthe SDS solution to make a solution of SDS encased SWNTs (SDS-SWNTs) forsubsequent analysis. FIG. 1 d illustrates the outer surface 130 of anSDS-SWNT iwrapped with SDS molecules 160.

A process to synthesize SDS-SWNTs is schematically illustrated in FIG.2. At step 210, an SDS solution is provided. At step 220, SWNTs areadded into the SDS solution. At step 230, a first sonication isperformed to disperse SWNTs to the SDS solution at a frequency in therange from 0 to 20 kHz for a time period from 0 to 5 minutes. At step240, a second sonication is performed to make an SDS encased SWNTssolution mixture at a frequency in the range from 20 to 200 kHz for atime period from 0 to 15 minutes. At step 250, the solution mixture iscentrifuged and only the supernatant that contains SDS-SWNTs iscollected. At step 260, the supernatant is diluted to make a solution ofSDS-SWNTs suitable for subsequent optical measurements.

SDS Encased SWNTs Response to pH Change

According to one embodiment of the present invention, the opticalproperties of SDS encased SWNTs are responsive to pH change in theirenvironment. As illustrated in FIG. 3 a, at pH 11, the UV/vis/NIRabsorption of SDS encased pristine Tube@Rice SWNTs has a prominent peak310 with wavelength range from 1400 nm to 2000 nm. The intensity of thepeak 320 corresponding to the pH in the solution at 3 is significantlylower than the intensity of the peak 310 at pH 11. Similar results wereobserved for SDS encased pristine HiPco SWNTs as illustrated in FIG. 3b. The UV/vis/NIR absorption of SDS encased pristine HiPco SWNTs has aprominent peak 330 with wavelength range from 1000 nm to 1600 nm. Theintensity of the peak 340 corresponding to the pH in the solution at 3is significantly lower than the intensity of the peak 330 at pH 11.

SWNTs Encased with Other Detergents

Charge groups are crucial for pH sensing of SWNTs. Three different kindsof detergents, sodium dodecyl sulfate (SDS) with charge groups, TritonX-100 and poly(vinylpyrrolidone) (PVP) without charge groups are chosenas coating materials for SWNTs. As illustrated in FIG. 4 a-d,respectively, the pristine Tube@Rice and HiPco SWNTs are insensitive topH changes under the coating of Triton X-100 and PVP polymer because thespectra obtained at pH 3 or 11 are nearly super imposable to each other.However, as shown in FIG. 3 a and 3 b, respectively, the pristineTube@Rice and HiPco SWNTs show pH-dependence after encased with SDS. Theimportance of charge group in SWNTs pH sensing has been confirmed byusing other materials with charge groups such as poly(sodium4-styrenesulfonate (PSS, MW ˜70,000). As illustrated in FIG. 5 a, at pH11, the UV/vis/NIR absorption of SDS encased pristine Tube@Rice SWNTshas a prominent peak 510 with wavelength range from 1400 nm to 2000 nm.The intensity of the peak 520 corresponding to the pH in the solution at3 is significantly lower than the intensity of the peak 510 at pH 11.Similar results were observed for SDS encased pristine HiPco SWNTs asillustrated in FIG. 5 b. The UV/vis/NIR absorption of SDS encasedpristine HiPco SWNTs has a prominent peak 530 with wavelength range from1000 nm to 1600 nm. The intensity of the peak 540 corresponding to thepH in the solution at 3 is significantly lower than the intensity of thepeak 530 at pH 11.

FIG. 6 listed the chemical structure of some polymers used as models forpH sensing according to the present invention. SDS and PSS havenegatively charged sulfate 610 and sulfonate 620 group, respectively.

The results indicate that the negatively charged groups on the coatingmaterials are necessary for SWNT-based pH sensing [14, 30, 48]. The pHrange for observation of the optical changes of SWNTs may differdepending on the encasing material used and its isoelectric point orequilibrium constants, which changes the pH range for protonation anddeprotonation. To achieve the optimal pH sensing range, other detergentsthat are analogs of or structurally similar to SDS or PSS can be used.Detergents with negative charge may also be used together as a mixture.

Pristine SWNTs do not respond to pH changes when they are dispersed inwater by using a neutral polymer surfactant Triton X-100 wrapping. Theobservation of no changes in the electronic band structure S₁₁ ofpristine SWNTs when they are exposed in neutral polymer surfactantenvironments suggests an important application for using those polymersas protecting regents when no perturbation for pristine SWNTs isrequired in aqueous solution.

SDS Encased SWNTs Response to Sonication in Aerated Water

Vigorous ultrasonication is often employed to enhance the dissolution ofSWNTs in SDS aqueous solution, by encasing individual nanotubes in SDS.Such isolated nanotubes are crucial for separating metallic andsemiconducting nanotube [20]. However, the sonication of aerated wateris a quite complex process that generates various reactive intermediates[36-41]. As illustrated in FIG. 7, nitrous acid 710 (HNO₂) and nitricacid 720 (HNO₃) are formed in aerated water under sonication. Theeffects of sonication on the optical properties of SWNTs are furtherstudied.

Experimental Examples

Pristine HiPco SWNTs, with tube diameters between 0.7 and 1.1 nm and alength distribution from several hundred nanometers to a fewmicrometers, were purchased from Carbon Nanotechnology, Inc [23]. SDSwas purchased from Sigma-Aldrich with purity >99%. HiPco solutions in 1wt % SDS in H₂O were prepared using methods known to people skilled inthe art [23]. Briefly, about 2.4 mg pristine HiPco SWNTs were weighed ona microgram-scaled balance in a TGA and placed in a 10 mL test tube with5 mL 1 wt % SDS aqueous solution. An ultrasonic bath (Branson Model1510R-MT, 42 kHz with rated power output of 70 W) was used to disperseHiPco SWNTs in SDS solutions. The starting ultrasonic bath temperaturewas room temperature and it may increase up to 40° C. under prolongedsonication. The mixture in the test tube was sonicated for about 4minutes and then was centrifuged (Sargent-Welch Scientific Co.) for 15minutes. About 2 mL of the top portion of the centrifuged sample wasdecanted and diluted with 6 mL SDS solution. Two SWNT solution sampleswith volumes of 0.5 and 5 mL were transferred into two 10 mL test tubesfor further sonication studies. 0.5 mL is the minimum volume that couldbe used for optical absorption measurements and 5 mL is suitable for pHmeasurements. It was observed that the effects of sonication on SWNTsolutions were related to the solution volume. The 0.5 mL solutionrequired much longer sonication time (up to 65.5 minutes) to exhibitoptical property changes similar to those in the 5 mL solution (0-5minutes). For the sake of clarity, the results reported here are mainlyfor the 5 mL SWNT solution, only some results of 0.5 mL SWNT solutionswere included. UV/vis/NIR absorption spectra were measured by using aPerkin-Elmer UV/Vis/NIR spectrometer. A quartz cell of 1 mm path lengthwas used for holding solutions. The absorption spectra of the samplesafter different sonication times were recorded and the SDS solution wasused as a reference for background subtraction. An Orion Model 420 pHmeter with a Ag/AgCl referenced Orion pH glass electrode was used tomeasure the pH of 5 mL samples of SDS solution after sonication forvarious times. All measurements were conducted at room temperature.

The processes to measure the optical response of SDS-SWNTs to pH changeinduced by ultrasonication is schematically illustrated in FIG. 8. Atstep 810, a solution of SDS-SWNTs is provided. At step 820, theUV/vis/NIR absorption and pH value of the solution are measured as basevalues. A first sonication is performed at step 830 on the SDS-SWNTssolution at a frequency in the range of from 20 to 200 kHz for a timeperiod of from 0 to 200 minutes at a temperature in the range of from 0to 100° C. The UV/vis/NIR absorption and pH value of the solution ismeasured at step 840 to find out the changes in absorption of SDS-SWNTsand in pH induced by the first sonication. A second sonication isperformed at step 850 on the SDS-SWNTs solution at a frequency in therange of from 20 to 200 kHz for a time period of from 0 to 200 minutesat a temperature in the range of from 0 to 100° C. The UV/vis/NIRabsorption and pH value of the solution is measured at step 860 to findout the changes in absorption of SDS-SWNTs and in pH induced by thesecond sonication. The sonication and measurement steps may be repeateduntil all desired data is collected.

Results and Discussion

FIG. 9 a shows the absorption spectra of the 5 mL SWNT solution aftersonication for times ranging from 0 to 3.5 minutes, with 0.5 minutesincrements. Spectra 905, 910, 915, 920, 925, 930, 935, and 940correspond to sonication time points 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0,and 3.5 minutes, respectively. The absorption bands at >830 nm come fromthe S₁₁ transitions of semiconducting SWNTs with different diameterswhile the bands at <830 nm belong to the interband transitions of thefirst pair (M₁₁) of metallic nanotubes and the second pair (S₂₂) ofsemiconducting nanotubes [5a,14,21,23-25,29,45]. The intensity of theS₁₁ bands decreases with sonication time while sonication has lessimpact on the M₁₁ and S₂₂ bands. Similar results were observed with 0.5mL SWNT solution as illustrated in FIG. 9 b. Spectra 945, 950, 955, 960,965, 970, 975, 980, and 985 correspond to sonication time points 0, 0.5,5.5, 10.5, 15.5, 20.5, 25.5, 35.5, and 45.5 minutes, respectively.Hardly any change in the spectra was observed after 0.5 minutessonication as it is shown that spectrum 945 from 0 minute measurement,i.e., no sonication, superimposed with spectrum 950 from 0.5 minutesmeasurement. The 0.5 mL solution required much longer sonication time(up to 65.5 minutes) to exhibit optical property changes similar tothose in the 5 mL solution (0-5 minutes). The observed changes in thespectra after the sonication are very similar to pH induced spectralchanges observed in Refs. 14, 18, and 24, an indication that the pH ofthe SWNT solution may drift into a more acidic range with sonication dueto the formation of nitric and nitrous acids by sonication [36-41].

To confirm that sonication induces changes in pH of the SWNT solution, acontrol experiment was conducted for direct pH measurements under thesame sonication conditions used for the SWNT solution in FIG. 9 a. Themeasured pH values are shown in FIG. 10, where a substantially linearrelationship is observed between sonication-induced pH and sonicationtime. Based on the result in FIG. 10, the S₁₁ region in FIG. 9 a isenlarged as shown in FIG. 11 a, with corresponding sonication-induced pHchanges. Spectra 905 (0 minute sonication), 910 (0.5 minute sonication),915 (1.0 minute sonication), 920 (1.5 minute sonication), 925 (2.0minute sonication), 930 (2.5 minute sonication), 935 (3.0 minutesonication), and 940 (3.5 minute sonication) correspond to pH values of6.1, 6.0, 5.9, 5.8, 5.6, 5.5, 5.3, and 5.2, respectively. The sonicationreduces the pH by only about 1 unit while significant spectral changesin the S₁₁ bands simultaneously occur, suggesting that SDS-encased SWNTsmay be used as a sensor for detection of sonolysis-induced pH changeswith high sensitivity [30, 14]. To further check the sonication-inducedpH decrease, pure water and SDS solutions in 1, 3, 5, 7, and 9 wt % werealso examined with 5 min sonication. A pH decrease in all solutionsamples was observed, indicating that it is a general phenomenon foraerated water [36-41]. In addition, after mixing an SWNT solution (thetop portion of the centrifuged SWNT sample with well-reserved S₁₁ bandintensities) with a 1 wt % SDS solution whose pH decreased to 5.0 aftersonication, a decrease in S₁₁ band intensities was also observed. Thisresult further indicates that the sonication-induced pH changes in thesolution may cause the spectral changes.

To further elucidate the relationship between the sonication-induced pHchanges and the interband intensity of S₁₁ bands, two representative S₁₁bands at 1245 nm and 1322 nm corresponding to semiconducting nanotubes(8, 7) and (9, 7) with diameters 1.03 and 1.1 nm, respectively wereselected [29]. The 1245 nm band may also overlap with bands from (9, 5),(10, 3) and (10, 5) nanotubes, and the 1322 nm band overlaps with bandsfrom (12, 4) and (13, 2) nanotubes [25,48]. The intensity of the twobands is plotted as a function of sonication-induced pH changes asillustrated in FIG. 11 b. There are nearly linear relationships betweenthe S₁₁ band 1245 NM (1110) and 1322 nm (1120) intensities andsonication-induced pH changes. The top axis 1130 shows the correspondingsonication time increasing from right to left.

It has been observed that the S₁₁ bands reversibly respond to pH changes[14, 18, 24, 25,48]. It was therefore unsurprising that increasing thepH recovered S₁₁ bands that were suppressed by sonication. FIG. 12 showsabsorption spectra of a 0.5 mL HiPco SWNT solution sonicated for 0.5minutes (1210) and 65.5 minutes (1220). After addition of 0.1 M NaOH toadjust the pH of the solution to 10, absorption of the S₁₁ bands werefully restored (1230). Lowering the solution pH to 5.0 by adding 0.1 MHCl suppresses the S₁₁ bands again, similar to other observations ofSDS-encased SWNTs [24].

The sonication of aerated water is a complex process involving variousreactive intermediates [36-41]. The above results suggest that the acidsgenerated by sonication may be responsible for the observed spectralchanges. There is also a consideration that degassing CO₂ duringsonication and redissolution of CO₂ after sonication may cause theobserved pH changes. A pH calculation was performed on water inequilibrium with CO₂ (assuming a maximum CO₂ concentration of 400 ppm)and it was found that the lowest pH attributable to atmospheric CO₂ wasabout 5.65 [49]. Therefore atmospheric CO₂ cannot bring the pH down to5.2 as shown in FIG. 10. The observed decrease in pH by sonication ismainly due to nitric acid and nitrous acid produced from the sonolysisof O₂, N₂ and H₂O as illustrated in FIG. 7 [36-41]. The protonation ofSDS-encased SWNTs induced by these acids depletes the valence bandelectrons of semiconducting SWNTs and so decreases the S₁₁ bandintensity [5a, 14]. However, the observed full restoration of the S₁₁bands by addition of NaOH also indicates that the actual latticestructure of the SWNTs is undisrupted after sonication.

To further examine whether the pH changes are the dominant factor in theoptical changes of SWNTs, an SDS-encased SWNTs in a 50 mM phosphatebuffer of pH 6.0 was also sonicated. The pH changes in a buffer shouldbe small and only subtle changes in optical absorption of sonicatedSWNTs are expected under these conditions. The results are shown in FIG.13. Spectra before (1310) and after (1320) 35.5 minutes sonication showthat prolonged sonication causes no significant spectral changes formost absorption bands except the 1322 nm band (1330), respectively. Theintensity of the 1245 nm band (1340) also decreases slightly. The resultis significantly different from non-buffered solutions as shown in FIG.9 a and 9 b, suggesting that the spectral changes of SWNTs undersonication may be mainly caused by a pH-related process. Other possibleintermediates such as oxidizing species NO and H₂O₂ [36-41] maycontribute to the spectral suppression through electron withdrawal fromSWNTs. However, as shown in FIG. 13, their contribution is more distinctfor the larger-diameter nanotubes such as the (9, 7) nanotubes at 1322nm and is less significant for the smaller-diameter nanotubes with bands<1170 nm without the involvement of the sonication-induced pH decease.

Conclusions

Ultrasonication is a necessary process to make single-walled carbonnanotubes (SWNTs) soluble in aqueous solution with surfactants such assodium dodecyl sulfate (SDS). It is observed that sonication induced pHchanges suppress the optical transitions of the first interbandtransition pair (S₁₁) in the density of states of semiconducting SWNTswhile other possible intermediates induced by sonication contribute lesssignificantly to the observed spectral changes without the involvementof sonication-induced pH decrease. The suppressed S₁₁ peaks can berestored by adding basic solution, suggesting that the lattice structureof SWNTs is undisrupted by the sonication used here. The absorbance ofS₁₁ peaks shows a nearly linear relationship with sonication induced pHchanges in the narrow pH range of 5.2 and 6.1. The results indicate thatSDS-encased SWNTs may be used as an indicator for sonolysis-relatedapplications.

Additionally, the results obtained suggest that to avoidsonication-induced optical changes of SWNTs, sonication can be done in abuffer with pH >6.0 or by making the SDS solution to more basic such aspH 10 as is usually done in sonication-related HiPco research. Example 2SDS encased SWNTs for Hydrogen Peroxide Sensing

SDS Encased SWNTs Respond to Hydrogen Peroxide Concentration

Many enzyme-catalyzed reactions yield hydrogen peroxide (H₂O₂) as aproduct [52]. Although H₂O₂ has been used in purification of SWNTs [7a],it is understood that no research is known to address its interactionwith SWNTs for optical sensing. In view of recent great progress in thedevelopment of nanolasers [33], nanowaveguides [53] and opticalnanofibers [34], there are urgent needs to develop new nanostructuralmaterials that can utilize these nano light sources for opticalnanosensors. Here, a representative ws-SWNT system, namely HiPco SWNTsencased in the surfactant sodium dodecyl sulfate (SDS) is used to studythe SWNT solution reaction with H₂O₂.

Experimental Examples

Raw HiPco SWNTs (purity ˜95 atm %) were purchased from CarbonNanotechnologies, Inc [23]. The nanotube diameters range from 0.7 to 1.1nm and are from several hundred nanometers to a few micrometers long[23]. The reagents SDS (>99% pure), H₂O₂ (30 wt %) and catalase (EC1.11.1.6, 1870 units/mg) were from Sigma-Aldrich. Nanotube solutions in1 wt % SDS in H₂O were prepared using a procedure known to peopleskilled in the art [23]. Briefly, in a typical experiment, about 2.4 mgpristine HiPco SWNTs were weighed on a microgram-scaled balance in a TGAand placed in a 10 mL test tube with 5 mL 1 wt % SDS aqueous solution.Mild sonication was applied in an ultrasonic bath (Branson Model 1510,42 kHz) for 1-3 minutes to disperse HiPco nanotubes, and then themixture was vigorously sonicated for about 1 minute. Longer sonicationtime was avoided since the optical features of SWNTs may be suppressedby prolonged sonication. The resulting mixture was centrifuged(Sargent-Welch Scientific Co.) for 1 hour, and then 0.8 mL of thesupernatant was decanted and diluted with the SDS solution. About 0.13mL of the diluted HiPco solution was mixed with an equal amount of pH6.0 phosphate buffer (50 mM) and transferred into a 1 mm quartz cell.The resulting solution had an optical absorbance of about 0.3 at 1245 nmat a HiPco concentration of about 0.1 mg/ml. A series of such solutionswere prepared with H₂O₂ concentrations ranging from 0 to 200 ppm. Thetime-dependent optical absorption of the solutions was measured at roomtemperature in 1 mm quartz cells using a Perkin-Elmer Lamda 19UV/Vis/NIR spectrometer [54]. All samples were prepared in pH 6.0 buffersolutions to eliminate possible pH induced spectral changes. SDSsolutions in pH 6.0 buffers without HiPco nanotubes were used forabsorption background subtraction. For reversibility experiments with pHtuning, 0.1 M NaOH and 0.1 M HCl solutions were used for pH titrations.An Orion Model 420 pH meter with a Thermo Electron Orion micro pH glasselectrode was used to measure the pH of the HiPco nanotube solutionsamples.

The process to measure the optical response of SDS-SWNTs to H₂O₂ atdifferent concentrations is schematically illustrated in FIG. 14. Atstep 1410, a solution of SDS-SWNTs is made or provided. At step 1420,the UN/vis/NIR absorption of the SDS-SWNTs solution is measured as abaseline. At step 1430, a first amount of H₂O₂ is added to the SDS-SWNTssolution and mix well to form a solution with a first concentration ofH₂O₂. UV/vis/NIR absorption of the solution is then measured at 1440 tofind out the change in absorption of SDS-SWNTs induced by the additionof the first amount of H₂O₂. At step 1450, a second amount of H₂O₂ isadded to the SDS-SWNTs solution and mix well to form a solution with asecond concentration of H₂O₂. UV/vis/NIR absorption of the solution ismeasured at 1460 to find out the change in absorption of SDS-SWNTsinduced by the addition of the second amount of H₂O₂. The process ofH₂O₂ addition and subsequent measurement may be repeated until all thedesired data are obtained.

Similarly the process to measure the optical response of SDS-SWNTs toH₂O₂ at different time points is schematically illustrated in FIG. 15.At step 1510, a solution of SDS-SWNTs is made or provided. At step 1520,the UN/vis/NIR absorption of the SDS-SWNTs solution is measured as abaseline. At step 1530, an amount of H₂O₂ is added to the SDS-SWNTssolution at time point t₁ and mix well to form a solution with a fixedconcentration of H₂O₂. UV/vis/NIR absorption of the solution is measuredat time point t₂ (1540) to find out the change in absorption ofSDS-SWNTs induced by the addition of the H₂O₂. UV/vis/NIR absorption ofthe solution is measured at time point t₃ (1550) to find out furtherchange in absorption of SDS-SWNTs induced by the addition of the H₂O₂.The process of time point measurement is repeated until all desired dataare obtained.

Results and Discussion

FIG. 16 shows a typical set of time-dependent absorption spectra ofSDS-encased HiPco SWNTs after addition of 30 ppm H₂O₂. The firstinterband transition of semiconducting SWNTs (S₁₁) occurs in the rangefrom 830 to 1360 nm, and the second interband transition ofsemiconducting SWNTs (S₂₂) occurs from 600 to 800 nm [21]. The peakintensity at wavelengths <700 nm or between 930 and 1040 nm isinsensitive to the presence of hydrogen peroxide. However, the spectralintensity decreases with time slightly between 700 and 930 nm anddramatically from 1040 to 1360 nm. Two most sensitive S₁₁ bands withlonger wavelengths, 1245 and 1322 nm, which correspond to largerdiameter nanotubes are the focus of the studies [29]. The spectralintensities are normalized to the intensity of a S₂₂ band at 659 nm. Theband at 1245 nm (1670) could come from (8, 7) nanotubes of 1.03 nm indiameter overlapping with bands of (9, 5), (10, 3) and (10, 5)nanotubes. The band at 1322 nm (1680) could be assigned to (9, 7)nanotubes with a diameter of 1.1 nm overlapping with bands of (12, 4)and (13, 2) nanotubes [25,29]. Note that the intensity of the S₁₁ bandsof larger diameter nanotubes at 1245 and 1322 nm decreases with theincreasing reaction time at 0, 15, 30, 60, 120, and 180 minutes,corresponding to spectra 1610, 1620, 1630, 1640, 1650, and 1660,respectively. The observed spectral changes suggest that there arestrong interactions between HiPco SWNTs and H₂O₂, probably as a resultof electron withdrawal from the valence band of the nanotubes by H₂O₂oxidation. The rate of the spectral changes increases with the H₂O₂concentration, indicating that the reaction is a diffusion-relatedprocess.

The H₂O₂ concentration dependence has been measured to determine thesensitivity. FIG. 17 a shows the absorption spectra of SDS-encased HiPcoSWNTs in pH 6.0 buffer solutions under H₂O₂ concentrations ranging from0 to 200 ppm. The spectra were taken 1 hour after mixing and thespectral intensities are normalized to the intensity of the S₂₂ band at659 nm, which is not sensitive to the presence of H₂O₂. The intensity ofthe S₁₁ bands decreases with the increase of H₂O₂ concentration. Thespectra 1705, 1710, 1715, 1720, 1725, 1730, 1735, and 1740 of SDS-SWNTsshow concentration dependence for different H₂O₂ concentrations 0, 1, 5,10, 30, 50, 100, and 200 ppm, respectively. The spectra are taken at thereaction time of 1 hour and the spectral intensities are normalized tothe intensity of the S₂₂ band at 659 nm. There is apparent intensitieserosion at 1245 nm and 1322 nm bands, as indicated by dashed lines 1745and 1750, respectively.

FIG. 17 b shows the H₂O₂ concentration-dependent absorbance of the twoS₁₁ bands of 1245 nm (1760) and 1322 nm (1755), plotted as the ratio ofAs₂₂/As₁₁. Both bands grow exponentially with H₂O₂ concentration. The1322 nm (1755) band change is saturated at 50 ppm (1765) with a linearrelationship from 1 ppm to 50 ppm (1770). The 1245 nm (1760) band changeis saturated at about 200 ppm (1775) and there is also a linearrelationship from 1 ppm to 50 ppm (1780). These concentration-absorbancerelationships may permit determination of the H₂O₂ concentration ofunknown solutions.

FIG. 18 shows the reversible optical response of a H₂O₂ interacted withSDS-SWNTs after removal of H₂O₂ by decomposing H₂O₂ into H₂O and O₂using MnO₂ catalyst. Data curve 1810 is the spectrum of SDS-SWNTs whenthere is no hydrogen peroxide present. Data curve 1830 is the spectrumof the same SDS-SWNTs when hydrogen peroxide concentration in thesolution is 1 ppm. Intensity suppressions are observed at bands 1245 nmand 1322 nm as indicated by dashed lines 1840 and 1850, respectively.When MnO₂, a known hydrogen peroxide decomposition agent, was added,however, the spectrum returned to the level before hydrogen peroxide wasadded as indicated by the grey dotted line 1820.

It has been shown that proteins can be immobilized on both oxidizedSWNTs and vacuum-annealed SWNTs [3d, 55]. Preliminary test indicatesthat HiPco nanotubes can be solubilized in aqueous enzyme solutions withthe assistance of mild sonication. To examine whether H₂O₂ is chemicallyreacting with SWNTs forming covalent bonds, an enzyme catalyst catalaseis chosen, because catalase is capable of decomposing H₂O₂ into H₂O andO₂. As shown in FIG. 19, as the catalase concentration increases from 0(data curve 1910) to 47 (data curve 1920), 94 (data curve 1930), and 140(data curve 1940), respectively, the S₁₁ bands shift to the red and the1114 nm band (1950) disappears, suggesting that the catalase coats theSWNTs by replacing some SDS molecules. Catalase therefore caneffectively decompose the H₂O₂. The spectral intensity reduction is dueto dilution by the added catalase.

As illustrated in FIG. 20, reversible optical response was observed whencatalase was used. Data curve 2010 is the spectrum of SDS-SWNTs whenthere is no hydrogen peroxide present. Data curve 2020 is the spectrumof the same SDS-SWNTs when hydrogen peroxide concentration in thesolution is 30 ppm. Significant intensity suppressions are observed atbands 1245 nm and 1322 nm. When catalase, a known hydrogen peroxidedecomposition agent, was added, however, the spectrum returned to thelevel before hydrogen peroxide was added as shown in spectrum 2030. Thecorresponding mechanistic stages are illustrated also. Configuration2040 illustrates an SWNT encased with SDS molecules. Configuration 2050illustrates SDS-SWNT interacts with hydrogen peroxide. And configuration2060 illustrates catalase interacts with SDS-SWNT in the present ofhydrogen peroxide, and converts hydrogen peroxide into water and oxygen.

The recoverable behavior indicates that there are no direct chemicalreactions (i.e. covalent bond formation) between the SWNT sidewalls andH₂O₂ under the conditions used here. Instead, the H₂O₂ may form chargetransfer complexes with SDS-encased HiPco nanotubes and thus cause theobserved spectral changes by altering the charge density on the SWNTs.H₂O₂ decreases the charge density of the SWNTs and so decreases theintensity of the S₁₁ bands [5a,5b,14]. Importantly, the observedrecoverability of HiPco nanotubes suggests the reusability of nanotubes,which is a highly desirable feature for nanotubes-based practical sensorapplications.

It is important to understand the mechanisms behind the SWNT spectralchanges induced by H₂O₂. There are at least four possible factors thatmight contribute to the changes: 1) the high reduction potential of H₂O₂[49]; 2) the possible reaction of H₂O₂ with SDS [56]; 3) electrostaticinteractions between negative charged SDS micelles and HO₂ ⁻ at high pHs[57]; and 4) the polar nature of H₂O₂ that may hinder this molecule'sapproach to SDS micelle-encased nanotubes. It is intuitive that the SWNTelectron withdrawal by H₂O₂ is related to its high reduction potential(standard reduction potential E₀=1.763 V) [49]. Since the potential ofH₂O₂ is tunable by changing pH, one might use pH changes to control itselectron withdrawing ability and thus the optical properties of theSWNTs. The reduction potential calculations using the Nernst equationshow that the reduction potential (E) of H₂O₂ is 1.33 V at pH 6.0 and aconcentration of 100 ppm (3.0×10⁻³ M). When the pH is increased to 10.0,E=1.09 V, so there is no significant decrease in the reduction potentialat higher pH. The potential E=1.09 V at pH 10.0 is still high enough toinduce charge transfer from the nanotubes to hydrogen peroxide.Therefore at even higher pH the H₂O₂ should still induce charge transferfrom nanotubes and suppress spectral features similar to those observedat pH 6.0. Surprisingly, an increase in the solution pH restores thesuppressed spectral features as shown in FIG. 21 a. The absorption of anSDS-SWNT solution containing 100 ppm H₂O₂ increases with increasing pHvalues of 6.3, 6.8, 7.1, 7.4, 7.8, 8.6 and 10.6, as illustrated byspectra 2110, 2120, 2130, 2140, 2150, 2160, and 2170, respectively.Spectrum 2180 is the control spectrum of SDS-SWNT alone, withouthydrogen peroxide. At pH 10.6 with corresponding spectrum 2170, theabsorption of the SDS-SWNT solution restores back to the absorptionlevel observed without the addition of 100 ppm H₂O₂ (2780).

Furthermore, the suppression and restoration is a reversible process asone tunes the pH up (2185) and down (2190) as shown in FIG. 21 b. Theratio As₁₁/As₂₂ of the absorbances of the S₁₁ (1322 nm) and the S₂₂ (659nm) peaks reversibly responds to the pH changes. This uniquepH-dependent optical property of the H₂O₂-SDS-HiPco complex suggestssome important attributes for optical pH sensing applications becausethe pH range (6-10) that the SDS-SWNT with H₂O₂ system is sensitive tois complementary to other pH sensing methods using SWNTs. In addition,the above result also indicates that the high reduction potential ofH₂O₂ may not play a significant role in the observed spectral changes.The other factors listed above may also contribute to the spectralchanges.

The process to measure the optical response of SDS-SWNTs to pH changesin the presence of fixed amount of H₂O₂ is schematically illustrated inFIG. 22. At step 2210, a solution of SDS-SWNTs is made or provided. InAt step 2220, the UV/vis/NIR absorption of the SDS-SWNTs solution ismeasured as a baseline value. An amount of hydrogen peroxide is added tothe solution of SDS-SWNTs and mixed well at step 2230. UV/vis/NIRabsorption of the solution is measured at step 2240 to find out thechange in absorption of SDS-SWNTs induced by the addition of H₂O₂. ThepH of the SDS-SWNTs solution containing H₂O₂ is adjusted to a firstvalue at step 2250. UV/vis/NIR absorption of the solution is measured tofind out the change in absorption of SDS-SWNTs solution containing H₂O₂at the first pH value at step 2260. The pH of the SDS-SWNTs solutioncontaining H₂O₂ is adjusted to a second value at step 2270. UV/vis/NIRabsorption of the solution is measured to find out the change inabsorption of SDS-SWNTs solution containing H₂O₂ at the second pH valueat step 2280. The pH adjustment and subsequent measurement are repeateduntil the spectrum returned to its original level.

Mechanistic Studies of the Response of SDS Encased SWNTs to HydrogenPeroxide

To elucidate SWNT interaction mechanisms with H₂O₂, control experimentswere carried out by choosing a mild oxidant, iodine. Iodine has arelatively low standard reduction potential of 0.620 V in the aqueous 12form and 0.536 V in the I₃ form [49]. In addition, because of thenon-polar characteristic of iodine, iodine molecules may more easilyapproach the SDS-encased nanotubes than H₂O₂. Two iodine aqueoussolutions were prepared to test the iodine reactions with SDS-HiPcosolutions; a 30 ppm aqueous iodine I₂ solution without I⁻ ions, and a 45ppm solution containing I₃ ⁻ (1.2×10⁻⁴ M) and I⁻ (2.4×10⁻² M) ions. Theiodine concentration is calculated as it is presented in the HiPcosolutions. The reduction potential of the later solution containing I₃ ⁻is calculated as 0.56 V, or about half of the H₂O₂ reduction potentialsindicated above. Addition of the iodine (in both forms of aqueous I₂ andI₃ ) into the SDS-HiPco solutions at pH 6.0 suppressed the nanotubespectral features within 10 minutes. Both I₂ and I₃ ⁻ produced the samespectral changes.

FIG. 23 shows the spectral changes of an SDS-HiPco solution caused by 30ppm I₂. After the addition of 30 ppm iodine the spectrum changedsignificantly from curve 2310 to curve 2320. The suppressed spectralfeatures are gradually restored by titrating the solution pH with 0.1 MNaOH to 7.1 (spectrum 2330), 7.8 (spectrum 2340), and 10.6 (spectrum2350). In contrast to H₂O₂, iodine also suppressed the spectral featuresbetween 930 and 1040 nm that are insensitive to H₂O₂. The suppressedfeatures were restored by increasing the pH to 10.6, just as occurredwith H₂O₂ as shown in FIG. 21.

The iodine results suggest that a high reduction potential may be notcritical for the observed SWNT spectral changes. There is also apossibility that H₂O₂ reacts with SDS to induce the nanotube spectralchanges [56]. However, the iodine results indicate that this is unlikelybecause iodine induces similar SWNT spectral changes even though it doesnot react with SDS under current conditions. Furthermore, thereversibility of the nanotube spectral changes upon removal of the H₂O₂with catalase or pH tuning suggests that the H₂O₂-SDS-nanotube system isquite stable. It is therefore likely that both H₂O₂ and iodine formcharge transfer complexes with HiPco nanotubes. The nonpolar nature ofiodine may permit this molecule to approach SDS-encased nanotubes morereadily than H₂O₂ and thus accelerate the spectral changes. Because ofthe random, structureless adsorption of SDS molecules on the sidewall ofnanotubes [58], the H₂O₂ molecules can access the nanotube surfaces bydiffusion through the micelles, causing relatively slow spectral changesinduced by charge transfer, as compared to those of iodine.

Recovery of the SWNT spectral features with pH increase may reflect SDSdeprotonation in more basic solutions, which changes the charge densityon the SDS-encased nanotubes by refilling the valence band [5a,5b,6,14]and strengthens the optical transitions. In addition, the increase in pHmay increase the electrostatic interactions because the H₂O₂ converts tonegatively charged HO₂ ⁻, which is repelled from negatively charged SDSmicelles [57]. The repulsion of hydrogen peroxide then eliminateselectron withdrawal from the nanotubes and restores their spectralfeatures. These two pH-related effects may work simultaneously so thesuppressed spectral intensity is restored or even strengthened as shownin spectrum 2170 of FIG. 21 a.

Conclusions

The near IR optical transitions of semiconducting SWNTs show that H₂O₂interacts with HiPco SWNTs through valence electron withdrawal, whichsuppresses the nanotube spectral intensity. The SWNTs respond opticallyto H₂O₂ at concentrations as low as 1 ppm. More intriguingly, thesuppressed nanotube band intensity recovers when the H₂O₂ is decomposedinto H₂O and O₂ with the enzyme catalyst catalase, or by increasing thesolution pH. The recoverability indicates that there are no directchemical reactions on the SWNT sidewall under these conditions.Preliminary studies on the mechanisms suggest that H₂O₂ may withdrawelectrons from the SWNT valence band by charge transfer, whichsuppresses the nanotube spectral intensity. These findings set a solidfoundation for H₂O₂ related optical sensing applications usingsurface-modified SWNTs.

SDS-encased, water-soluble SWNTs are optically sensitive to H₂O₂ andthus have potentially important applications for sensing of biologicalspecies. With recent inventions of nanolasers [33] nanowaveguides [53]and optical nanofibers [34], the nanotubes might be combined with thesenano light sources for development of nanotube-based optical nanosensorsin miniature devices. The optical properties of nanotubes thus hold outgreat promise for the accelerated realization of optical nanosensors.The findings also suggest enzyme-assisted molecular recognitionapplications by selective optical detection of biological species whoseenzyme-catalyzed products include hydrogen peroxide.

SDS Encased SWNTs Respond to Oxidants

Similar to H₂O₂, the optical response of SDS-SWNTs to iodine can also beused for sensing applications. Water soluble oxidants such as FeCl₃,AgNO₃, CuCl₂, (NH₄)₂(Ce(NO₃)₆), and (IrCl₆)²⁻ can also be exploited.

Example 3 SDS Encased SWNTs for Glucose Sensing

SDS encased SWNTs with glucose oxidase response to glucose concentrationIt has been found that nanotubes encased in the surfactant sodiumdodecyl sulfate (SDS) optically respond to hydrogen peroxide atconcentrations as low as 1 ppm. This finding suggests an optical sensingmethod for important biological molecules such as glucose, sincenumerous enzyme-catalyzed reactions produce hydrogen peroxide (H₂O₂)[52]. For example, glucose reacts catalytically with water and oxygen inthe presence of the enzyme glucose oxidase (GO_(x)) to produce

hydrogen peroxide and gluconic acid [52]. Therefore GO_(x)-modifiedSWNTs should respond optically to glucose. In the present invention, itis demonstrate that water-soluble SDS-encased SWNTs treated with GO_(x)can be used to optically detect glucose at concentrations as low as 0.25mM.

The optical measurements were conducted under two sets of experimentalconditions: (1) in pH 6.0 buffer solutions where only the product H₂O₂causes SWNT spectral changes and (2) non-buffer solutions where bothproducts H₂O₂ and gluconic acid are involved in the changes.

Experimental Example

Raw HiPco SWNTs (purity ˜95 atm %) were purchased from CarbonNanotechnologies, Inc [23]. Anhydrous α-D(+)-glucose (99+ % pure) waspurchased from Fisher Scientific. SDS (>99% pure), GO_(x) (EC 1.1.3.4,200 units/mg) and catalase (EC 1.11.1.6, 1870 units/mg) were fromSigma-Aldrich. A method known to people skilled in the art [23] wasemployed to prepare HiPco solutions in 1 wt % SDS in H₂O.

About 2.4 mg pristine HiPco SWNTs were weighed on a TGA microgrambalance and placed in a 10 mL test tube with 5 mL 1 wt % SDS aqueoussolution. In an ultrasonic bath (Branson Model 1510, 42 kHz), mildsonication was applied for 1-3 minutes to disperse HiPco nanotubes andthen the mixture was vigorously sonicated for about 1 minute. Shortsonication time was applied because the optical properties of HiPcoSWNTs are very sensitive to sonication. The resulting mixture wascentrifuged (Sargent-Welch Scientific Co.) for 1 hour. 0.8 mL of the topportion of the centrifuged sample was decanted and diluted with the SDSsolution.

For the buffer conditions, about 0.13 mL of the HiPco solution was mixedwith an equal amount of a pH 6.0 phosphate buffer (50 mM) in a 1 mmquartz cell. The resulting diluted solution has an optical absorbance ofabout 0.3 at 1245 nm with a HiPco concentration of about 0.1 mg/mi.GO_(x) in a pH 6.0 buffer was added into the solution with aconcentration of 40-80 units/mL. For the non-buffer conditions, thesolution preparation procedure was the same except that the 1 wt % SDSsolution was used for dilution instead of the pH 6.0 buffer. A series ofsuch solutions were prepared with glucose concentrations from 0 to 2 mM.The time-dependent optical absorption of the solutions in 1 mm quartzcells was measured with a Perkin-Elmer Lambda 19 UV/Vis/NIR spectrometerat room temperature. SDS solutions in water or in pH 6.0 buffers withoutHiPco nanotubes were used as a reference for absorption backgroundsubtraction. An Orion Model 420 pH meter with a Thermo Electron Orionmicro pH glass electrode was used to measure the pH of the solutions.

A process to measure the optical response of GOx-SDS-SWNTs to glucose atdifferent time points is schematically illustrated in FIG. 24. At step2410, a solution of SDS-SWNTs is made or provided. At step 2420, anamount of glucose oxidase is added to the SDS-SWNT solution to make aGOx-SDS-SWNTs solution. The UN/vis/NIR absorption of the GOx-SDS-SWNTssolution is measured as a baseline at step 2430. At step 2440, an amountof glucose is added to the GOx-SDS-SWNTs solution at time point t₁ andmix well to form a solution with a fixed concentration of glucose.UV/vis/NIR absorption of the solution is measured at time point t₂ atstep 2450 to find out the change in absorption of GOx-SDS-SWNTs inducedby the addition of the glucose. UV/vis/NIR absorption of the solution ismeasured at time point t₃ at step 2460 to find out further change inabsorption of GOx-SDS-SWNTs induced by the addition of the glucose. Theprocess of time point measurement is repeated until all desired data isobtained.

A process to measure the optical response of GOx-SDS-SWNTs to glucose atdifferent concentrations is schematically illustrated in FIG. 25. Atstep 2510, a solution of GOx-SDS-SWNTs is made. At step 2520, theUN/vis/NIR absorption of the GOx-SDS-SWNTs solution is measured as abaseline. At step 2530, a first amount of glucose is added to theGOx-SDS-SWNTs solution and mix well to form a solution with a firstconcentration of glucose. UV/vis/NIR absorption of the solution ismeasured at step 2540 to find out the change in absorption of SDS-SWNTsinduced by the addition of the first amount of glucose. At step 2550, asecond amount of glucose is added to the GOx-SDS-SWNTs solution and mixwell to form a solution with a second concentration of glucose.UV/vis/NIR absorption of the solution is measured at step 2560 to findout the change in absorption of GOx-SDS-SWNTs induced by the addition ofthe second amount of glucose. The process of glucose addition andsubsequent measurement is repeated until data for all desired glucoseconcentrations are obtained.

The process to measure the optical response of GOx-SDS-SWNTs to pHchanges in the presence of fixed amount of glucose is more specificallyillustrated in FIG. 26. At step 2610, a solution of GOx-SDS-SWNTs ismade. At step 2620, the UV/vis/NIR absorption of the GOx-SDS-SWNTssolution is measured as a baseline value. An amount of glucose is addedto the solution of GOx-SDS-SWNTs and mixed well at step 2630. UV/vis/NIRabsorption of the solution is measured to find out the change inabsorption of GOx-SDS-SWNTs induced by the addition of glucose at step2640. The pH of the GOx-SDS-SWNTs solution containing glucose isadjusted to a first value at step 2650. UV/vis/NIR absorption of thesolution is measured to find out the change in absorption ofGOx-SDS-SWNTs solution containing glucose at the first pH value at step2660. The pH of the GOx-SDS-SWNTs solution containing glucose isadjusted to a second value at step 2670. UV/vis/NIR absorption of thesolution is measured to find out the change in absorption ofGOx-SDS-SWNTs solution containing glucose at the second pH value )2680.The pH adjustment and subsequent measurement are repeated until thespectrum returned to its original level.

Results and Discussion

It has been demonstrated that proteins can be immobilized on the surfaceof SWNTs [3d,55]. FIG. 27 a shows the spectrum of the SDS-nanotubesolution before (curve 2710) and after (curve 2715) the addition ofGO_(x). Spectra 2710 and 2715 look almost the same except that the threenear IR peaks at 1170 nm (peak 2720), 1245 nm (peak 2730) and 1322 nm(peak 2740) shift to the red by about 3-8 nm to 1178 nm (peak 2725),1252 nm (peak 2735) and 1324 nm (peak 2745), respectively, afteraddition of GOx. In addition, the band 2750 at 1114 nm becomes ashoulder 2755. These observed spectral changes suggest that GO_(x) coatsthe SWNTs.

Addition of 0.5 mM glucose changes the spectral intensity with time asshown in FIG. 27 b. Spectra 2760, 2765, 2770, 2775, 2780, 2785, and 2790correspond to time points 0, 0.2, 1, 2, 3, 4, and 14 hours,respectively. The first interband transition of semiconducting SWNTs(S₁₁) is in the near IR range from 830 to 1360 nm and the secondinterband transition of semiconducting SWNTs (S₂₂) ranging from 600 to830 nm [18, 23, 24, 29]. The S₁₁ bands originate from individualnanotubes with different diameters and chiralities [29]. To correct thedilution effect on the spectral intensity, the spectra in FIG. 27 arenormalized to the S₂₂ band at 659 nm, selected because it is insensitiveto H₂O₂ or pH changes. The most sensitive S₁₁ bands 2795, 2797 at 1245nm and 1322 nm correspond to (8, 7) and (9, 7) nanotubes, with 1.03 nmand 1.1 nm diameters, respectively [29]. The 1245 nm band may overlapwith bands of (9, 5), (10, 3) and (10, 5) nanotubes, and the 1322 nmband overlaps with bands of (12, 4) and (13, 2) nanotubes [29]. Theintensity of these S₁₁ bands significantly decreases with the reactiontime, as was observed in the SDS-HiPco solutions reacted with H₂O₂.Since the pH changes are minimal in a pH buffer, the observed spectralchanges are probably caused by the hydrogen peroxide produced from theglucose oxidase catalyzed glucose reaction.

It was observed that the H₂O₂-suppressed spectral features can berestored by decomposing H₂O₂ into H₂O and O₂ with enzyme catalase or byincreasing pH to more basic. Two additional experiments were conductedto verify that H₂O₂ induced the observed spectral changes in FIG. 27 b.FIG. 28 a shows that catalase restores the spectral features of theglucose-reacting nanotube solution. Curve 2810 is the spectra ofSDS-SWNT in pH 6.0 buffers with 40 units/mL GOx. Curve 2820 is thespectra of SDS-SWNT in pH 6.0 buffers with 40 units/mL GOx after theaddition of 0.5 mM glucose. Curve 2830 is the spectra of SDS-SWNT in pH6.0 buffers with 40 units/mL GOx after the addition of 0.5 mM glucosewith 140 units/mL catalase. Similarly, adjusting the pH of theglucose-interacted nanotube solution from 6.0 to 10.0 restores thesuppressed features to what they were before glucose was added as shownin FIG. 28 b. Curve 2840 is the spectra of SDS-SWNT in pH 6.0 bufferswith 40 units/mL GOx. Curve 2850 is the spectra of SDS-SWNT in pH 6.0buffers with 40 units/mL GOx after the addition of 0.5 mM glucose. Curve2860 is the spectra of SDS-SWNT in pH 6.0 buffers with 40 units/mL GOxafter the addition of 0.5 mM glucose with pH adjusted to 10 by additionof 0.1 M NaOH. These results are similar to those caused by H₂O₂ andsuggest that the product H₂O₂ is responsible for the observed spectralchanges in the GO_(x)-SDS-HiPco nanotubes in pH 6.0 buffers.

Gluconic acid is another product of the reaction. The acid lowers the pHof an unbuffered solution. SDS-encased HiPco nanotubes are opticallysensitive to pH changes [24], so both H₂O₂ and the acid-induced pHchanges may contribute to the SWNT spectral suppression in unbufferedsolutions. This dual effect enhances the sensitivity of GO_(x)-SDS-HiPconanotubes to glucose. FIG. 29 a shows the concentration-dependentabsorption spectra of GO_(x) immobilized HiPco nanotubes in 1 wt % SDSsolutions (pH ˜6.5) in non-buffer conditions. The positions of the twoS₁₁ bands of (8,7) and (9,7) nanotubes shift from 1245 and 1322 nm to1254 and 1328 nm, respectively with addition of 80 units/mL GO_(x). Thespectra are taken 30 min after glucose is added. Spectra 2910, 2920,2930, 2940, and 2950 correspond to glucose concentrations 0, 0.25, 0.5,1, and 2 mM, respectively. The intensity of the S₁₁ bands decreases withthe increase of glucose concentration. FIG. 29 b shows the normalizedabsorbance 2960, 2970 corresponding to the two bands at 1254 nm and 1328nm versus glucose concentrations, respectively. The intensity of the1254 nm band 2960 has a linear relationship with the concentrations. Theintensity of the 1328 nm band 2970 decays exponentially with glucoseconcentration. The observed concentration dependence may permitdetermination of unknown glucose concentrations.

The observed sensitivity of the surface-modified nanotubes to glucosemay be sufficient for designing devices such as near IR optical fibersensors [59,60] for detecting glucose under physiologically relevantconditions, e.g. in blood where the glucose concentration is in therange of 1-20 mM [60]. The spectral changes occur more rapidly at higherglucose concentrations as shown in FIG. 30, so glucose detection will bequicker in the physiological concentration range. In FIG. 30 a, changesin the absorption spectra of GO_(x)-SDS-nanotubes in a 1 wt % SDSsolution as a function of time after addition of 1.0 mM glucose isshown. Spectra 3010, 3020, 3030, 3040, and 3050 correspond to timepoints 0, 10, 30, 60, and 120 minutes, respectively. The GO_(x)concentration is 80 units/mL. The spectral intensities are normalized tothe intensity of the S₂₂ band at 659 nm. FIG. 30 b shows normalizedabsorbance of the S₁₁ band at 1328 nm, plotted as the ratio As₁₁/As₂₂ ofthe peaks S₁₁ (at 1328 nm) and S₂₂ (at 659 nm) of GO_(x)-SDS-HiPconanotubes in 1 wt % SDS solution, changes as a function of time afteraddition of various glucose concentrations. The solid lines are fittingcurves that follow exponential decays. Curves 3060, 3070, 3080, and 3090correspond to glucose concentrations 0.25, 0.50, 1.0, and 2.0 mM,respectively. The absorbance decreases more quickly at higher glucoseconcentrations. The results further suggest that molecularrecognition-based nanotube optical sensors could be combined with nanolight sources [33, 34, 53] to create new device designs for nanoscalemedical and clinical applications.

Conclusion

Water-Soluble single-walled carbon nanotubes (SWNTs) coated with glucoseoxidase (GO_(x)) and sodium dodecyl sulfate (SDS) exhibit an opticalresponse to glucose at different concentrations. The GO_(x) catalyzedglucose reaction produces H₂O₂ and gluconic acid. Both products depleteelectrons from the valence band of SWNTs through charge transfer withH₂O₂ or protonation of the encasing SDS molecules, resulting in thesuppression of the SWNT spectral intensity. The peaks S₁₁ and S₂₂correspond to the first and second interband transitions, respectively,in the density of states of semiconducting SWNTs with differentdiameters.

It is demonstrated for the first time that SDS-encased SWNTs immobilizedwith GO_(x) can be used to optically detect glucose at concentrations aslow as 0.25 mM. This sensitivity to glucose is sufficient for practicalglucose sensing under physiological conditions. Nanotube-based opticalsensors for molecular recognition can be integrated with recentlydeveloped nano light sources to create new approaches for nanoscalemedical and clinical applications.

SDS Encased SWNTs with Glucose Oxidase and Iodide Response to Glucose

In the GOx-SDS-HiPco solutions, an amount of iodide (I⁻) can also beadded. The GOx catalyzed reaction produces H₂O(₂, which will react withI⁻H₂O₂+2I⁻+2H⁺=2H₂O+I₂to produce iodine. It is know that I₂ reacts with nanotubes veryeffectively, so the sensitivity and response time of glucose sensing maybe improved greatly.SDS Encased SWNTs with Other Enzymes

Because SDS-SWNTs are sensitive to hydrogen peroxide, they can be usedto sense a broad range of enzyme substrates whose correspondingenzymatic turnover produce hydrogen peroxide as one of the majorproducts. Some known enzyme-substrate examples that produce hydrogenperoxide as one of their major products include uricase-uric acid,alcohol oxidase-ehthanol, cholesterol oxidase-cholesterol, and lactateoxidase-lactate etc.

Example 4 Preparation of Double-Stranded DNA Wrapped SWNTs and Analysisof Their Physical Properties

In another aspect of the present invention, HiPco SWNTs can be dispersedinto double-stranded DNA aqueous solutions to form stable solutions. Thefirst optical interband transitions of the DNA wrapped semiconductingHiPco SWNTs possess a unique pH dependence, a phenomenon observed inSDS-encased and carboxylate group functionalized SWNTs.

Experimental Example

Double-stranded DNA from salmon testes was purchased from Sigma-Aldrich(catalog number D1626). Pristine HiPco SWNTs were purchased from CarbonNanotechnologies, Inc. The samples were weighed on a microgram-scaledbalance in a TGA. The solutions were prepared by a method known topeople skilled in the art [27]. About 0.40 mg of DNA was added in 5.0 mLpH 7.0 Tris buffer or 10 mM TE buffer. The DNA was dissolved in thebuffer by sitting overnight or by sonicating for approximately 5 minutesin an ultrasonic bath (Branson Model 1510, 42 kHz) at 0° C. A colorlesstransparent solution 3104 is formed and the picture of the solution isshown in FIG. 31 a. 3.40 mg of HiPco nanotubes was then added into the 5mL DNA solution and the mixture was sonicated for about 1 hour at 0° C.A black stable DNA wrapped HiPco SWNTs (DNA-SWNTs) solution 3108 isformed and the picture of the solution is shown in FIG. 31 a. It hasbeen observed that the DNA-SWNTs solution is stable for more than oneyear in a refrigerator. MicroRaman measurements were conducted on thesamples using a ReniShaw MicroRaman 1000 Spectrometer with 532 nm laserexcitation from a frequency-doubled Nd:YAG laser [28]. The spectra werecalibrated with diamond single crystal and Si standards.

For pH dependence study, a DNA-SWNTs solution was titrated with 0.1 MNaOH from pH 7.0 to pH 10.0 with increments of 0.5. To test thereversibility of the pH dependence of the DNA-SWNTs sample, the pH 10.0solution was titrated with 0.1 M HCl from pH 10.0 down to 6.0, and thenback-titrated to pH 9.0 with 0.1 NaOH. The pH changes during titrationswere monitored with an Orion Model 420 pH meter. UV/vis/NIR absorptionspectra of the solution at different pHs were measured by using aPerkin-Elmer Lambda 19 UV/vis/NIR spectrometer. A quartz cell of 1 mmpath length was used for holding solutions. A DNA solution without HiPconanotubes was prepared as a reference. The DNA solution was titrated tothe same pH as the DNA-SWNTs solution for background subtraction.

Results and Discussions

FIG. 31 b illustrates Raman spectra of a pristine HiPco mat sample 3120and a DNA-SWNTs sample 3110 drop-dried on a Si substrate. The Sisubstrate has excitation wavelength of 532 nm 3130. The spectra werenormalized using the main tangential mode at 1588 cm⁻¹ 3140. The Ramanspectra of pristine 3120 and DNA-SWNTs 3110 show roughly the samefeatures including the radial breathing mode 3150 (RBM) at about 150-300cm⁻¹, the tangential modes 3160 at 1500-1600 cm⁻¹ and the disorder (D)mode 3170 at 1330 cm⁻¹ [26, 28]. In comparison with the pristine HiPcosample, the D mode in the DNA-SWNTs sample shows little change inintensity, indicating that in the DNA-SWNTs sample, no significantdefects were introduced in the sonication assisted dissolution process.

The optical absorption spectra of individual SWNTs in aqueous solutionshave distinct features that provide “fingerprints” for theidentification of different nanotubes [23-25]. FIG. 32 a shows theabsorption spectra of the DNA-SWNTs solution at pH values of 7.0(spectrum 3210), 7.5(spectrum 3215), 8.0(spectrum 3220), 8.5(spectrum3225), 9.0(spectrum 3230), 9.5(spectrum 3235) and 10.0(spectrum 3240),respectively. The absorption bands 3245 (>900 nm ) come from the firstinterband transition S₁₁ of semiconducting SWNTs of different diameters.The larger diameter nanotubes show bands centered at longer wavelengthbecause the interband transition energy is approximately inverselyproportional to the nanotube diameter [29]. The bands <900 nm belong tothe interband transitions of the first pair M₁₁ of metallic nanotubes ataround 440-650 nm (band 3250) and the second pair S₂₂ (band 3255) ofsemiconducting nanotubes [21, 29]. In FIG. 32 a, the spectral intensitydecreases due to dilution with 0.1 M NaOH. For clarity, the spectra areplotted in the same scale but shifted with equal division for clarity ofillustration. When pHs are changed, there are no significant changes inthe shape or intensity of the M₁₁ and S₂₂ bands. However, the S₁₁ bandsof larger diameter nanotubes at 1190 nm (band 3260) and 1280 nm (band3265) become more distinguishable and intensify with increasing pH. Theband at 1190 nm could be assigned to (11, 3) nanotubes with a diameterof 1.01 nm overlapping with bands of (8, 6) and (12, 1) nanotubes, andthe band at 1280 nm could come from (8, 7) nanotubes of 1.03 nm indiameter overlapping with bands of (9, 5), (10, 3) and (10, 5) nanotubes[25, 29]. The intensity of the two bands is suppressed at pH 7.0, butrecovers when the pH is raised and the solution becomes more basic.

To investigate the relationship between pH and the intensity of the S₁₁bands, the absorbance of the (8, 7) band at 1280 nm is plotted as afunction of pH, as shown in FIG. 32 b. To correct for the dilutioneffect on absorbance, the intensity of the band at 1280 nm, 3265 in FIG.32 a, is normalized to the 730 nm band, 3270 in FIG. 32 a, one of theS₂₂ bands that are insensitive to pH changes. The normalized absorbanceA_(S) ₁₁ (1280)/A_(S) ₂₂ (730) increases monotonically with pH in therange between 7.5 and 10.0. This range is wider than that observed byDekker's group where the authors built an SWNTs based single-moleculeelectronic device [30]. They observed that when redox enzyme glucoseoxidase (GO_(x)) was attached to the nanotube sidewall, the SWNTs devicesenses pH changes in the studied pH range from 4.0 to 5.5. The observedpH dependent behavior of the DNA-SWNTs samples resemble that observedfor SWNTs functionalized with carboxylic groups [14] or encased in SDS[24], thus suggesting that the DNA serves as a pH sensing group.Specifically, the phosphate groups on the DNA deprotonate when pH isincreased. The deprotonation of DNA-encased SWNTs may refill the valenceband of semiconducting SWNTs with electrons, so the relative interbandintensity of the S₁₁ bands increases [5a].

The deprotonation and protonation processes appear to be reversible. Toexamine whether the pH-dependent optical response is reversible, the pH10.0 DNA-SWNTs solution was back-titrated with 0.1 M HCl, and the resultis shown in FIG. 33 a, with spectra 3305, 3310, 3315, 3320, 3325, 3330,3335, 3340, 3345 and 3350 correspond to pHs 5.5, 6.0, 6.5, 7.0, 7.5,8.0, 8.5, 9.0, 9.5 and 10.0, respectively. The overall spectralintensity decreases with the decrease of pH due to the dilution fromadded HCl solution. The interband transitions of the first pair M₁₁ ofmetallic nanotubes at around 440-650 nm (band 3355) and the second pairS₂₂ (band 3360) of semiconducting nanotubes appear unchanged. Theabsorption bands >900 nm come from the first interband transition S₁₁(band 3365) of semiconducting SWNTs. When the pH decreases to 6.0, thebands 3370 at 1190 and 1280 nm are suppressed and becomeindistinguishable, same for spectrum 3375 at pH 5.5. The observedfeatures at low pHs are very similar to the features observed in theinitial solution at pH 7.0 (band 3275) in FIG. 32 a. It is noted thatthe protonation process shown in FIG. 33 a does not exactly follow thedeprotonation process shown in FIG. 32, i.e. there is some hysteresisduring the back titration. To further verify that these two bands arerecoverable by deprotonation, a third titration is performed by additionof NaOH solution to titrate from pH 6.0 up to 9.0 and the result isshown in FIG. 33 b. The spectra are plotted on the same scale butshifted with equal division for clarity of illustration, with spectra3302, 3304, 3306, 3308, 3312, and 3314 correspond to pHs 6.0, 6.5, 7.0,7.5, 8.0 and 9.0, respectively. The overall spectral intensity decreaseswith the decrease of pH due to the dilution from added HCl solution. Theinterband transitions of the first pair M₁₁ of metallic nanotubes ataround 440-650 nm (band 3316) and the second pair S₂₂ (band 3318) ofsemiconducting nanotubes appear unchanged. The absorption bands >900 nmcome from the first interband transition S₁₁ (band 3322) ofsemiconducting SWNTs. When pH increases, the suppressed 1190 and 1280 nmfeatures reappear as indicated along dotted lines 3324 and 3326,respectively, indicating their intensities increase with increasing pHfor the DNA-SWNTs sample, same as the first deprotonation process shownin FIG. 32 a, indicated along dotted lines 3260 and 3265, respectively.

In comparison with the SDS-encased HiPco SWNTs samples, which have arelative narrow pH sensitive range of about 5.0-6.0 [24], the DNA-SWNTssample shows a pH dependent behavior at pH range above 6.0. The shift inthe pH range may depend on the coating materials used whose isoelectricpoints or equilibrium constants contribute to the difference in pH rangefor protonation and deprotonation. In addition, it is observed that thecharged groups on the coating materials are important for SWNTs based pHsensing, a conclusion also made by Dekker's group [30]. In anotherembodiment, single stranded DNA can be used to encase SWNTs for chemicalcompound sensing applications.

While there has been shown several and alternate embodiments of thepresent invention, it is to be understood that certain changes can bemade as would be known to one skilled in the art without departing fromthe underlying scope of the invention as is discussed and set forthabove and below including claims. Furthermore, the embodiments describedabove and claims set forth below are only intended to illustrate theprinciples of the present invention and are not intended to limit thescope of the invention to the disclosed elements.

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1. A method for surface modification of single walled carbon nanotubes,comprising the steps of: a. providing a detergent solution; b. adding aplurality of single walled carbon nanotubes into the detergent solution;c. performing a first sonication to disperse the single walled carbonnanotubes in the detergent solution; and d. performing a secondsonication after the first sonication to make detergent encased singlewalled carbon nanotubes, wherein at least one of the plurality of singlewalled carbon nanotubes is at least partially wrapped by one or moredetergent molecules to make it a detergent encased single walled carbonnanotube.
 2. The method of claim 1, wherein the detergent comprises SDS,PSS or a combination of them.
 3. The method of claim 1, wherein thefirst sonication process is performed at a frequency in the range offrom 0 to 20 kHz for a time period of from 0 to 5 minutes.
 4. The methodof claim 1, wherein the second sonication process is performed at afrequency in the range of from 20 to 200 kHz for a time period of from 0to 15 minutes.
 5. The method of claim 1, wherein each of the first andsecond sonication processes is performed at a frequency for a timeperiod such that no significant amount of defects that may affect theoptical properties of the single walled carbon nanotubes is introduced.6. The method of claim 1, wherein at least one optical property of thedetergent encased single walled carbon nanotubes responds to a chemicalproperty change in the solution of the detergent encased single walledcarbon nanotubes.
 7. The method of claim 6, wherein the single walledcarbon nanotubes comprise semiconducting nanotubes, metallic nanotubesor a combination of them.
 8. The method of claim 7, wherein the responseof the at least one optical property of the detergent encased singlewalled carbon nanotubes to the chemical property change of the solutionof the detergent encased single walled carbon nanotubes is moresensitively related to the semiconducting nanotubes than the metallicnanotubes in the solution of the detergent encased single walled carbonnanotubes.
 9. The method of claim 6, wherein the response of the atleast one optical property of the detergent encased single walled carbonnanotubes to the chemical property change of the solution of thedetergent encased single walled carbon nanotubes is reversible.
 10. Abiosensor responsive to a chemical property in an environment,comprising: a. a plurality of single walled carbon nanotubes forming anarray and showing a dependence of the chemical property, wherein atleast one of the plurality of single walled carbon nanotubes is at leastpartially wrapped by one or more detergent molecules to make it adetergent encased single walled carbon nanotube; and b. a processorcoupled to the array of the plurality of single walled carbon nanotubesfor processing the response of the plurality of single walled carbonnanotubes to the chemical property.
 11. The biosensor of claim 10,wherein the detergent comprises SDS, PSS or a combination of them. 12.The biosensor of claim 10, wherein the chemical property is a hydrogenperoxide concentration in an environment, and the detergent encasedsingle walled carbon nanotube is optically responsive to the hydrogenperoxide concentration in the environment.
 13. The biosensor of claim12, wherein the at least one detergent encased single walled carbonnanotubes is further wrapped by one or more enzyme molecules to form asolution of detergent encased single walled carbon nanotubes with theenzyme.
 14. The biosensor of claim 13, wherein the hydrogen peroxide maybe produced by an enzyme as one of the turnover products from acorresponding substrate.
 15. The biosensor of claim 10, wherein thechemical property is glucose concentration in an environment, thedetergent encased single walled carbon nanotube is further wrapped byone or more glucose oxidase that may covert the glucose to hydrogenperoxide and gluconic acid, and the at least one detergent encasedsingle walled carbon nanotube with glucose oxidase is opticallyresponsive to hydrogen peroxide that is produced from the glucose byglucose oxidase in the environment.
 16. A surface modified single walledcarbon nanotube, comprising, a. a single walled carbon nanotube that hasa layer of carbon atoms forming a wall defining a cavity therein,wherein the wall as formed has an outer surface and an inner surface,and a first end and an opposite, second end; and b. at least onemolecule non-covalently attached at least to one of the inner surfaceand the outer surface of the single walled carbon nanotube, wherein thesingle walled carbon nanotube is at least partially surface modifiedwith the at least one molecule to show an optical dependence of achemical property of an environment.
 17. The surface modified singlewalled carbon nanotube of claim 16, wherein the single walled carbonnanotube comprises one of a semiconducting nanotube and a metallicnanotube.
 18. The surface modified single walled carbon nanotube ofclaim 17, wherein the dependency of the chemical property of thesurfaced modified single wall nanotube shown optically is moresensitively related to the semiconducting nanotube than the metallicnanotube.
 19. The surface modified single walled carbon nanotube ofclaim 16, wherein the chemical property dependence of the surfacemodified single walled carbon nanotube is reversible.
 20. The surfacemodified single walled carbon nanotube of claim 16, wherein the at leastone molecule comprises one of SDS, glucose oxidase, single stranded DNA,double-stranded DNA and PSS.
 21. The surface modified single walledcarbon nanotube of claim 16, wherein the chemical property is one of pHvalue, hydrogen peroxide concentration, glucose concentration andethanol concentration of the environment.
 22. A method of detecting achemical compound, comprising the steps of: a. providing a solution ofsurface modified single walled carbon nanotubes, wherein each of thesurface modified single walled carbon nanotubes is at least partiallywrapped by one or more detergent molecules to make it soluble; b.associating the solution of surface modified single walled carbonnanotubes with the chemical compound; and c. detecting optically achemical property change of the solution of surface modified singlewalled carbon nanotubes corresponding to the chemical compound so as todetect the chemical compound.
 23. The method of claim 22, wherein thedetergent comprises SDS, PSS or a combination of them.
 24. The method ofclaim 22, wherein the associating step comprises a step of forming asolution of the surface modified single walled carbon nanotubes and thechemical compound.
 25. The method of claim 22, wherein the chemicalcompound comprises at least one of a base and acid, and thecorresponding chemical property is pH of the solution of the surfacemodified single walled carbon nanotubes.
 26. The method of claim 22,wherein the chemical compound is hydrogen peroxide, and thecorresponding chemical property is hydrogen peroxide concentration inthe solution of the surface modified single walled carbon nanotubes. 27.The method of claim 22, further comprising the step of adding an amountof glucose oxidase to the solution of the surface modified single walledcarbon nanotubes before the associating step so that at least one of theplurality of surface modified single walled carbon nanotubes is furtherwrapped by one or more glucose oxidase molecules.
 28. The method ofclaim 27, wherein the chemical compound is glucose, and thecorresponding chemical property is glucose concentration in the solutionof the surface modified single walled carbon nanotubes with glucoseoxidase.
 29. The method of claim 28, wherein the glucose oxidase mayconvert glucose to hydrogen peroxide and gluconic acid, and theoptically detecting step comprises a step of measuring the opticalproperties of the solution of the surface modified single walled carbonnanotubes with glucose oxidase responsive to the concentration of thehydrogen peroxide that is produced from glucose by glucose oxidase inthe solution of the surface modified single walled carbon nanotubes withglucose oxidase.
 30. The method of claim 22, wherein the method furthercomprises the step of adding an amount of enzyme to the solution of thesurface modified single walled carbon nanotubes before the associatingstep so that at least one of the plurality of surface modified singlewalled carbon nanotubes is further wrapped by one or more of the enzymemolecules.
 31. The method of claim 30, wherein the chemical compound isan substrate of the enzyme that is convertable to hydrogen peroxide asone of its turnover products by the enzyme, and the correspondingchemical property is the substrate concentration in the solution of thesurface modified single walled carbon nanotubes with the enzyme.
 32. Themethod of claim 22, wherein the chemical compound is iodine, and thecorresponding chemical property is the iodine concentration in thesolution of the surface modified single walled carbon nanotubes.
 33. Themethod of claim 22, wherein the chemical compound is oxidant, and thecorresponding chemical property is the oxidant concentration in thesolution of the surface modified single walled carbon nanotubes.
 34. Themethod of claim 22, before the associating step, further comprising thesteps of: a. adding an amount of glucose oxidase to the solution of thesurface modified single walled carbon nanotubes so that at least one ofthe surface modified single walled carbon nanotubes is further wrappedby one or more glucose oxidase molecules; and b. adding an amount ofiodide to the solution of the surface modified single walled carbonnanotubes with glucose oxidase.
 35. The method of claim 34, wherein thechemical compound is iodine that is produced in situ from the reactionof iodide with hydrogen peroxide, which is produced from glucose by theglucose oxidase, and the chemical property is the iodine concentrationin the solution of the surface modified single walled carbon nanotubes.36. A method of optically detecting a chemical property change in asolution of surface modified single walled carbon nanotubes induced bysonication, comprising the steps of: a. providing a solution of surfacemodified single walled carbon nanotubes, wherein each of the surfacemodified single walled carbon nanotubes is at least partially wrapped byone or more detergent molecules to make it soluble; b. performing asonication on the solution of surface modified single walled carbonnanotubes; and c. detecting optically the response of the solution ofsurface modified single walled carbon nanotubes to a chemical propertychange of the solution of the solution of surface modified single walledcarbon nanotubes induced by the sonication.
 37. The method of claim 36,wherein the detergent comprises SDS, PSS or a combination of them. 38.The method of claim 36, wherein the sonication is performed at afrequency in the range of from 20 to 200 kHz for a time period of from 0to 200 minutes at a temperature in the range of from 0 to 100° C. 39.The method of claim 36, wherein the chemical property is pH in thesolution of surface modified single walled carbon nanotubes.
 40. Themethod of claim 39, wherein the chemical property change iscorresponding to nitrous acid and nitric acid concentrations induced bysonication in the solution of surface modified single walled carbonnanotubes.